This section provides background information related to the present disclosure which is not necessarily prior art.
Diesel engines produce an exhaust that contains nitrogen oxides (NOx), which is a regulated pollutant. NOx can be removed from the exhaust using a process known as selective catalytic reduction (SCR), which utilizes ammonia (NH3) as a chemical reductant to react with the NOx and form nitrogen (N2) on the surface of an SCR catalyst. The ammonia utilized in SCR is derived from a diesel exhaust fluid (DEF), which is a mixture of urea and water that is dosed into the engine exhaust stream. When the engine exhaust has sufficient enthalpy (temperature and flow rate), the water of the DEF is readily evaporated and the urea is decomposed to ammonia as the DEF is dosed into the engine exhaust stream. If the engine exhaust does not have sufficient enthalpy, however, the water is not as readily evaporated and the urea does not decompose to a sufficient extent, which can lead to the development of solid deposits within the exhaust conduit. This typically occurs when the ambient environment is cold, or when the engine has not been operated for an extended period of time. In either case, the exhaust after-treatment system has not had a sufficient amount of time to be heated by the engine exhaust and develop the sufficient amount of enthalpy to avoid the water not being readily evaporated and the urea not decomposing to a sufficient extent.
One solution that has been proposed for the above-noted problem is to modify an exhaust after-treatment system to include a low-temperature SCR catalyst at a location upstream of a diesel oxidation catalyst (DOC). A low-temperature SCR catalyst is able to achieve NOx conversion to nitrogen at lower temperatures (i.e., at cold start, or in cold weather) sooner than a SCR catalyst component located downstream of the DOC due to the low-temperature SCR catalyst component receiving nearly all of the exhaust enthalpy that exits the engine. Unfortunately, there are several potential disadvantages or trade-offs associated with the use of a low-temperature SCR component upstream of the DOC.
Firstly, because the low-temperature SCR component must also receive ammonia to convert the NOx to nitrogen, the after-treatment system may require a first DEF dosing module or injector that is specifically designated for dosing DEF for use by the low-temperature SCR component, and a second DEF dosing module or injector that is specifically designated for dosing DEF for use by the primary SCR component located downstream of the DOC. The additional dosing module or injector increases the complexity and cost of the after-treatment system.
Secondly, an ammonia slip catalyst (ASC) must be located between the low-temperature SCR component and the DOC to prevent or at least substantially minimize ammonia that slips through the low-temperature SCR component from reaching the DOC and being oxidized to NOx or N2O.
Thirdly, in systems where the low-temperature SCR component is designed to accept an entirety of the exhaust flow from the engine, it is likely that the low-temperature SCR component will need to be sized too large to be located immediately downstream from the engine (i.e., too large to be close-coupled, or located in the engine compartment). In contrast, such a design would require that the low-temperature SCR component be located further downstream from the engine and immediately upstream from the DOC. As a result, the exhaust may lose a significant amount of enthalpy as it travels through the lengthened section of the exhaust passage, which may negate the benefits of the low-temperature SCR component.